This inventon relates to the transmission and reception of color television.
In the United States, color broadcasts are made according to National Television Systems Committee (NTSC) composite standards, which require that picture information be separated into two components: luminance, or brightness, and chrominance, or color. FIG. 1 is an amplitude-vs.-frequency diagram illustrating in simplified form a typical NTSC composite color television signal 10 comprising a luminance signal 12 and a chrominance signal 14. (A composite television signal is one in which chrominance information is carried on a subcarrier. Other composite signals are SECAM, which is used in France, and PAL, which pre-dominates in the rest of Europe.) The signal occupies a nominal bandwidth of 6 MHz with the picture carrier 16 being 1.25 MHz above the lower end of the band. Luminance information is modulated directly onto picture carrier 16, while chrominance information is modulated onto color subcarrier 18 which is in turn used to modulate picture carrier 16. Color subcarrier 18 has a frequency of 3.579545 MHz, a standard established by the NTSC. (Audio information is carried on another subcarrier 20 lying near the upper edge of the band.)
The region labelled A in FIG. 1 is of particular importance, for it represents overlap between the luminance 12 and chrominance 14 signals. Since separation of luminance and chrominance is accomplished by filtering a frequency-division multiplexed signal, overlaps such as A between the two signals lead to several problems. If, upon reception, complete separation between luminance and chrominance is desired, the necessary filtering will cause the loss of some of the information in both signals. On the other hand, if no loss of information can be tolerated, then one must accept interference between the luminance and chrominance signals. Moreover, since the various parts of the NTSC television signal are transmitted at different frequencies, phase shifts occurring during transmission will affect them differently, causing the signal to deteriorate. Also, the available color information is severely limited by the small color bandwidth permitted.
To overcome these problems, a system called Multiplexed Analog Components (MAC) has been developed. The MAC color television signal is illustrated in FIG. 2, which is an amplitude-vs.-time diagram of a single video line of 63.56 us duration. The first 10.9 us is the horizontal blanking interval (HBI) 22, in which no picture information is transmitted. Following HBI 22 are chrominance signal 24 and luminance signal 26, either of which may be time-compressed. Between chrominance signal 24 and luminance signal 26 is a 0.28 us guard band 28, to assist in preventing interference between the two signals.
The MAC color television signal of FIG. 2 is obtained by generating conventional luminance and chrominance signals (as would be done to obtain a conventional NTSC or other composite color television signal) and then sampling and storing them separately. Luminance is sampled at a luminance sampling frequency and stored in a luminance store, while chrominance is sampled at a chrominance sampling frequency and stored in a chrominance store. The luminance or chrominance samples may then be compressed in time by writing them into the store at their individual sampling frequency and reading them from the store at a higher frequency. A multiplexer selects either the luminance store or the chrominance store, at the appropriate time during the active video line, for reading, thus creating the MAC signal of FIG. 2. If desired, audio samples may be transmitted during the HBI; these are multiplexed (and may be compressed) in the same manner as the video samples. The single rate at which all samples occur in the multiplexed MAC signal is called the MAC sampling frequency.
Although the MAC format of FIG. 2 overcomes the problems of the composite television signal of FIG. 1, these have been replaced by several other difficulties. One physical embodiment of a prior art MAC system having acceptable picture quality uses a luminance sample frequency of 13.50 MHz, compressing luminance in the ratio of 3:2. The MAC sample frequency is therefore 20.25 MHz. However, the NTSC horizontal line frequency f.sub.H (for a 63.56 us line) is 0.01573 MHz, which is 1/1287th of the MAC sample frequency used in this system. Therefore, since all three frequencies must be generated in each of the many receivers which would receive this signal, complicated and expensive frequency-generation circuitry must be used at each receiver to regenerate the color subcarrier and other necessary frequencies, adding greatly to the cost of such a system. (All composite color television signals use a color subcarrier frequency of 227.5 f.sub.H ; however, f.sub.H in each system is different. Therefore, only in an NTSC signal will the 227.5 f.sub.H color subcarrier have a frequency of 3.579545 MHz.)
Another physical embodiment of a prior art MAC system uses a luminance sample frequency of 10.74 MHz (682.5 f.sub.H) and compresses luminance in the ratio of 4:3. The MAC sample frequency is therefore 14.32 MHz (910 f.sub.H). This system, in addition to the complicated frequency-generation circuitry required in the receivers, has unacceptable picture quality because of the low luminance sample frequency and, therefore, cannot satisfy the commercial requirements and Nyquist criterion for desired luminance bandwidth.
A third physical embodiment of a prior art MAC system uses a MAC sample frequency of 21.48 MHz (1365 f.sub.H) and a luminance compression ratio of 5:4. Luminance sampling is therefore done at 17.18 MHz (1092 f.sub.H), which does provide acceptable picture quality. However, once again, complicated frequency-generation circuitry is required. Furthermore, if both luminance and chrominance are to fit into an active video line of the same length as an NTSC active video line (52.38 us), chrominance must then be compressed in the ratio 5:1. The chrominance signal quality is then unacceptable because of noise. A variation of this prior art system compresses chrominance in the ratio 15:4 to avoid the noise problem; but this simply adds to the complexity of the frequency-generation circuitry and extends luminance and chrominance beyond the active video line into the horizontal blanking interval, with the result that insufficient time remains in each video line for the necessary audio information.
Adding to the complexity of frequency generation is the requirement that a color subcarrier at a frequency of 227.5 f.sub.H (3.579545 MHz for NTSC) be generated at each receiver. A technique for easily generating the subcarrier without high cost is required. Because standard receivers are constructed to receive composite television signals, they cannot directly receive a MAC television signal. The MAC signal used for transmission must first be converted to a composite television signal (such as NTSC, PAL, or SECAM) at the receiver.